Formulated ceramic resistor compositions are widely used in thick film resistor electrical parts, thick film hybrid circuits, etc. They are compositions for preparing a resistive thick film by printing the composition on a conductor pattern or other electrodes formed on the surface of an insulating substrate, followed by firing the print to form the resistor.
The thick film resistor composition is prepared by dispersing a conducting component and an inorganic binder in an organic medium (vehicle). The conducting component, such as ruthenium oxide, inorganic matrix material, such as inorganic glasses, and organic medium component are mixed and deposited on the substrate by many known methods. Following the fusion of the deposited layer, the choice of inorganic and conductive components largely determines the electrical properties of the thick film resistor. The inorganic binder comprises glass, and has a major role of retaining the thick film integrally and binding it to the substrate. The organic medium is a dispersing medium that affects the application properties, particularly the rheology, of the composition.
Traditional thick film resistors have relied on the use of lead-containing glasses. In addition, lead ruthenate (PbRuO3) conductive oxide is often employed in resistors with sheet resistivity of at least 1000 ohm/sq, and especially 10,000 ohm/sq and higher. On the other hand, there is increasing environmental concern around the use of lead in commercial products, so a high quality Pb-free resistor system is desirable.
U.S. Pat. No. 7,481,953 to Tanaka, et al takes an approach focused on the addition of BaTiO3 and Ag to a CaO based glass composition and a ruthenium containing conductive material to form a substantially lead-free resistor composition.
Commonly assigned U.S. Pat. No. 5,491,118 to Hormadaly discloses a cadmium-free and lead-free thick film composition suitable for resistors and thermistors. Bi2O3 containing glasses are used, which provide a high and negative TCR. Also, Hormadaly discloses that the addition of MgO, Nb2O5 and TiO2 TCR drivers is to be avoided for their deleterious effects on resistance and also the stability of the resultant pastes.
Thus, when making substantially lead-free resistors, the challenge is to provide novel glass chemistries that must work with substantially lead-free conductive oxides. Because lead ruthenate cannot be used, developing a substantially lead-free system is particularly difficult for resistor values above approximately 1000 ohm/square.
The difficulty is not limited to just the resistance but also extends to the temperature coefficient of resistance (TCR) being held within ±100 ppm/° C. Both hot TCR (HTCR) and cold TCR (CTCR) are usually reported, with HTCR typically being measured between room temperature and 125° C., while CTCR between room temperature and −55° C. The elimination of Pb from the resistor requires novel glass chemistries to control both resistivity and TCR, either individually or in combination.